Fortunately, I brought my sketch book with me and across from my booth was a couple of park benches. When someone sat down, I donned my sunglasses (so they wouldn't see me STARING at them haha) and sketched away.
Those that moved to fast or got up as soon as they sat down, turned more into doodles but there's nothing wrong with that either.
I'm currently working on a commission for a woman who is a returning customer. Her friend's dog, Glover, recently crossed the rainbow bridge and she's getting a pet portrait of him. He was such a chunky dog, but so cute! She said that he loved his fluffy chew toys and would always greet you with one in his mouth, so I kept one in the portrait. (it'll be between his front legs, still white paper here)....